The present invention relates to nacelles associated with turbofan engines mounted to aircraft, and more particularly, to a fan duct of such a nacelle constructed to suppress fan noise and minimize turbulence.
The nacelle for a high bypass ratio turbofan engine used on a transport aircraft performs a number of critical functions. It supports the engine within an outwardly streamlined housing and plays a vital role in determining the efficiency of the engine, both of which affect the overall performance of the aircraft. The gas turbine engine together with the associated nacelle design are also very important in determining the noise footprint of the aircraft in the vicinity of airports. The thrust reverser portion of the nacelle performs the additional task of slowing the aircraft after it touches down on the runway. The nacelle must perform these critical functions under substantial mechanical loads, temperature extremes and vibrational forces with minimum weight and complexity.
In order to meet Governmental noise regulations it is conventional to line the fan inlet and exhaust duct walls of a turbofan nacelle with a noise absorbing material. A honeycomb noise attenuation structure sold under the trademark DynaRohr has been extensively used for this purpose, an example of which is disclosed in U.S. Pat. No. 4,379,191 of Beggs et al. assigned to Rohr Industries, Inc. This structure includes a core having a multiplicity of open cells sandwiched between an outer imperforate facing sheet and an inner perforated sheet with holes that communicate with the cells. A microporous sheet material such as finely woven stainless steel cloth is bonded over the perforated sheet and forms a part of the inwardly exposed surface of the duct. The degree of porosity and the fineness of the microporous sheet, the size and spacing of the holes in the perforated sheet, and the size of the cells in the core are critical to the absorption characteristics, and in particular, to the range of frequencies that can be effectively absorbed.
The air inlet of a turbofan nacelle comprises a large cylindrical inlet duct which is spaced forwardly of the fan. On rotation of the aircraft during take-off, the nacelle inlet is placed in an inclined attitude. The inlet duct has a generally airfoil-shaped cross-section with a relatively thick leading edge portion. This configuration reduces turbulence within the inlet and reduces fan degradation that would lessen engine thrust during the critical take-off period. Unfortunately, the thick leading edge portion of the nacelle inlet is not needed during cruise and it adds significant drag. This drag adversely affects the fuel economy of the aircraft.
In the past, various boundary layer control schemes have been devised for improving laminar flow in aerodynamic applications. For example, slotted and otherwise perforated wings and control surfaces have been provided. During flight a thin boundary layer of turbulent air can be eliminated by sucking it through the slots. See for example U.S. Pat. No. 3,516,895 of Hartman. U.S. Pat. No. 2,853,852 of Bodine, Jr. discloses Helmholtz resonators extending through the wall of a jet engine inlet duct for attenuating aerodynamic-acoustic vibrations within the duct. U.S. Pat. No. 3,820,628 of Hanson discloses boundary layer control for annular flow splitters and radial inlet guide vanes positioned within the inlet duct of a turbine driven fan. The object of the disclosed design is to suppress noise in the inlet duct and to minimize drag and turbulence caused by insertion of such splitters and vanes. The splitters and vanes have slotted surfaces and interior cells designed to form acoustic chambers. The boundary layer of air is sucked through the slotted surfaces, into the chambers and through manifolds formed within the splitters and vanes by a central suction pump connected to the manifolds.